This invention relates generally to non destructive flaw detection in metals and more particularly to such flaw detection in and around fasteners without the need to remove the fastener.
As a matter of routine safety inspection, aircraft and other vehicles and machines are inspected for fatigue and damage during periodic maintainence checks. Many systems of inspection are used including penetrating dyes, ultrasonics and magnetic field generator-detectors. Any particular system may have a particular type of test situation where it is more efficient then another systems. However, until now there has never been an acceptable system that could test fastener holes without first removing the fastener.
A fastener, whether it is a bolt, rivet or some other similar device has a head which covers an area on or near the surface of the material. This head has precluded non destructive testing for cracks or other fatigue or accidentally induced damage. In the past it was always necessary to remove the fastener.
The time and labor involved in removing a fastener increases the cost of inspection substantially. A way has been long sought to enable safe, accurate non destructive testing to take place in the case of fastener holes.
One prior art device suggests the use of the Hall Effect to measure the intensity of an induced magnetic flux directly. This technique requires two costly sensing elements along with a complex probe structure. Further, this type of system has shown to be less reliable and accurate then the eddy current floating probe proposed herein. Hall Effect type devices are sensitive to thickness of the structural member, localized variations in metallurgy, electrical or physical characteristics of the material, surface variations which affect the distances between the Hall devices and surface of the material being inspected, and temperature variations.